The optical transmission characteristics of an electrochromic (EC) device are controlled by the amount of charge imparted to the device. Specifically, increasing the amount of charge in an electrochromic device results in a "darkening" or "coloring" of the device, while decreasing the amount of charge in an electrochromic device results in a "lightening" or "bleaching" of the device. Since an electrochromic device may be modeled as a non-linear passive device having an impedance dominated by a capacitive component, the amount of charge imparted to an electrochromic device is typically controlled by utilizing current sources and current sinks.
Electrochromic control arrangements utilizes circuit components having characteristics that tend to drift over time and temperature, thereby imparting more or less charge to the EC device than is otherwise desired. To ensure that an appropriate charge level is imparted to an electrochromic device, some electrochromic control arrangements electrically or optically measure the device directly to determine the present charge level and, if necessary, adjust that charge level. Such measurement is necessary because of the inability to predict with a sufficient degree of certainty the degradation of components within a particular system.
It is seen to be desirable to provide an electrochromic control apparatus that corrects for system and device errors within an electrochromic control system without directly measuring the electrochromic device. Moreover, it is seen to be desirable to provide such electrochromic control apparatus in an energy efficient manner.